1. Field of the Invention
This invention relates to an apparatus for measuring the luminous lifetime of samples, and more particularly, to the improvements in a multichannel Time-to-Amplitude Converter (hereinafter referred to as a "TAC") which constitutes the principal part of the apparatus.
2. Description of the Prior Art
The measurement of the luminous lifetime of samples is usually made by detecting, with the aid of a photo-multiplier tube, for example, the fluorescent pulse being emitted from a sample which is excited by a pulse of light, and by observing the time lapse from the leading edge of the pulse-excited light until the fluorescent pulse is emitted. In this case, since the mere measurement of time is unfit for the later treatment of signals, it is necessary to convert the time detected to an amplitude proportional to the above-mentioned time lapse from the leading edge of the pulse-excited light until the fluorescent pulse is emitted, wherefore the "TAC" is employed. Incidentally, the measurement of the luminous lifetime can be simplified sharply if it would be possible to measure all of the plurality of luminous pulses being emitted by a single excitation and to convert the measurements from time to amplitude. A multichannel "TAC" is capable of measuring all of a plurality of luminous pulses excited in such a way and of converting the measurements from time to amplitude. This multichannel "TAC" is provided with many channels (8 channels in the illustrated example in FIG. 1) such as "TAC" 1 consisting of a flip-flop FF.sub.1, a constant-current regulated power supply I.sub.1, a charging capacitor C.sub.1, a switch SW.sub.1 for regulating the changeover of the charging current to the capacitor C.sub.1, and a sample and hold circuit SH.sub.1 for sampling and holding the final charging voltage of the capacitor C.sub.1. Each of the flip-flops FF.sub.1, FF.sub.2 . . . is set by the start pulse P.sub.A being emitted at the leading edge of the pulse-excited light. When the flip-flops FF.sub.1, FF.sub.2 . . . are set, the switches SW.sub.1, SW.sub.2 . . . are turned ON, and the capacitor C.sub.1, C.sub.2 . . . start charging, that is, when the respective start pulse P.sub.A is supplied, all of the channels start the time-to-amplitude conversion. Next, when the luminous pulse P.sub.B1 used as the stop pulse is supplied, the flip-flop FF.sub.1 in the first channel is reset, and the switch SW.sub.1 is turned OFF. The flip-flops of the remaining channels are not reset by a first single stop pulse because the gate circuits G.sub.1, G.sub.2 . . . are inserted in the respective reset input lines.
By the above-mentioned SW.sub.1 being turned OFF, the charging of the capacitor C.sub.1 is stopped, and the time-to-amplitude conversion is ended and the charging voltage is held by the sample and hold circuit SH.sub.1. On the other hand, by the flip-flop FF.sub.1 being reset, the gate circuit G.sub.1 provided on the reset input of the second channel is enabled. According, the second stop pulse is supplied to flip-flop FF.sub.2 of the second channel so as to reset it, and the switch SW.sub.2 is turned OFF; at the same time, the gate circuit G.sub.2 on the reset input of the third channel is also enabled. By the switch SW.sub.2 being turned OFF, the charging of the capacitor C.sub.2 is ended and the charging voltage is held by the sample and hold circuit SH.sub.2.
Hereinafter, every time, whenever the third and fourth, etc., stop pulses are supplied, the flip-flops FF.sub.3, FF.sub.4 . . . are successively reset, and the time-to-amplitude conversions are stopped one after another. Since the output voltages of the "TACs" in the respective channels are the final charging voltages of the respective capacitors, as shown in FIG. 2, they are voltages which are proportional to the time lapses from the times when the start pulses P.sub.A have been supplied until the respective stop pulses P.sub.B1, P.sub.B2 . . . are supplied.
Hereupon, in the above-mentioned multichannel "TAC", although it is not possible for any "TAC" to start the conversion instantaneously when the start pulse P.sub.A is inputted thereto, and to stop the conversion instantaneously when its corresponding stop pulse is inputted, yet in practice there are some differences in length among the wires to each of the flip-flops FF.sub.1, FF.sub.2 . . . , and in the lag time of each of the gate circuits. Since the time intervals from the start pulses to the stop pulses are on the order of nanoseconds, there arises a problem that in each of the channels, the time interval after the conversion starts until it comes to an end does not exactly correspond to the time interval from the start pulse to the stop pulse, the interval being accompanied by an error which varies from channel to channel. This problem may be grasped, in other words, as a problem in that the time axis of each channel gets out of position due to the time delays of the wiring or of the elements. The time axis error of this type can be eliminated by inserting the delay elements .tau..sub.1, .tau..sub.2, .tau..sub.3 . . . on the input side of the "TAC" in every channel, as shown in FIG. 1 and by thereby individually regulating the delay time in every element. However, even with such means, the following defects remain:
1--Each delay element must continuously vary the delay time; however, such a delay element is very expensive, particularly for use with high speed pulses.
2--Although it is preferable to bring the setting knob of the delay element to the front panel of the system, because there is a necessity of properly regulating the delay time, it is impossible to electrically extend the delay element as far as the front panel and away from the circuitboard since high-speed pulses are passed through the delay element. For this reason, there is no choice but to use a cumbersome unreliable method as to mechanically extend the shaft of the delay element control knob, leaving the delay element mounted on the circuitboard.